The present invention relates to a method of correcting the temperature characteristics of a flow velocity sensor for measuring the flow rate of a gas.
Flow velocity sensors having various structures have been proposed to measure the flow rate of a gas. For example, as one of such sensors, a thermal flow velocity sensor based on the semiconductor manufacturing techniques is disclosed in Japanese Patent Laid-Open No. 60-142268.
As shown in FIG. 9, in this thermal flow velocity sensor, a thin film bridge portion 3 is formed on the surface of a semiconductor substrate 1 through a concave gap portion 2 for thermally insulating the bridge portion 3 from the substrate 1. A heater element 4 is formed at a middle portion of the surface of the bridge portion 3, and temperature-measuring resistive elements 5 and 6 for heat detection are formed on both sides of the heater element 4 in such a manner that the elements 4, 5, and 6 float above the substrate 1.
When a current is supplied to the heater element 4 of the flow velocity sensor having such an arrangement to generate heat, and a gas moves from a direction 8 indicated by an arrow while the sensor is placed in the flow of the gas, the temperature-measuring resistive element 5 on the upstream side is cooled by the flow of the gas to cause a decrease in temperature, whereas the temperature of temperature-measuring resistive element 6 on the downstream side is increased by the gas heated by the heater element 4.
As a result, a temperature difference occurs between the upstream and downstream temperature-measuring resistive elements 5 and 6, and their resistances change. For this reason, if the upstream and downstream temperature-measuring resistive elements 5 and 6 are incorporated in a Wheatstone bridge circuit, and changes in resistance are converted into voltages, a voltage output corresponding to the flow velocity of the gas can be obtained. As a result, the flow velocity of the gas can be detected. The flow rate of the gas can be immediately obtained by multiplying the detected flow velocity of the gas by the cross-sectional area of a portion through which the gas flows.
If, however, the flow velocity sensor having the above arrangement is vertically arranged to measure the flow rate of a gas with the upstream and downstream temperature-measuring resistive element 5 and 6 being respectively placed at upper and lower positions, the elements 5 and 6 are influenced by natural convection. More particularly, if the ambient temperature is lower than a normal temperature such as room temperature, since the density of a target gas to be measured is increased, heat generated by the heater element 4 tends to be easily transferred to the temperature-measuring resistive elements 5 and 6, and the upstream temperature-measuring resistive element 5 is heated more. For this reason, the output from the flow velocity sensor shifts to the negative side from a measurement value at a normal temperature such as room temperature with no flow of the gas.
In contrast to this, if the ambient temperature is higher than a normal temperature, since the density of a target gas is decreased, heat generated by the heater element 4 tends not to be easily transferred to the temperature-measuring resistive elements 5 and 6, and the upstream temperature-measuring resistive element 5 is cooled. For this reason, the output from the flow velocity sensor shifts to the positive side from a measurement value at a normal temperature such as the room temperature with no flow of the gas. Consequently, stable measurement cannot be performed.